1. Field of the Invention
The present invention relates to a cooling device for laser diodes.
2. Description of the Related Art
Forced water-cooling mechanisms and micro fluid channel water-cooling mechanisms are widely used as cooling mechanisms for a block on which a laser diode is mounted. The former can ensure its heat transport capacity by employing a pipe with a larger diameter and increasing its flow rate. The diameter of the pipe, however, needs to be at least 3 mm to reduce the internal frictional resistance and to ensure an increased water flow rate. In addition, installation of a water-cooling section inside the block inevitably results in the water-cooling section being separated from the microscopic light-emitting spot of the laser diode by a distance of about 2 mm. Thus, it is difficult for the cooling section to cool the area in the vicinity of the microscopic light-emitting spot of the laser diode. In contrast, the latter can adopt a design utilizing a fluid channel of a smaller diameter, so that it can cool the area in the vicinity of the light-emitting spot of the laser diode. But it can not provide a large flow volume due to the smaller diameter of the fluid channel. It is contemplated, therefore, to provide an increased flow rate by increasing the working pressure of the coolant. However, the smaller diameter of the fluid channel inevitably has a limitation with regard to its flow rate. Increased flow rate results in heat generation by the viscosity resistance friction against the inner wall of the fluid channel, resulting in a reduced cooling effect. Thus, for a high-power infrared bar laser of the 60 W class, in which the amount of heat dissipation is about 100 W, the heat dissipation density provided by the water-cooling mechanism is limited to about 1 kW/cm2.
Meanwhile, a heat pipe composed of a decompressed airtight tubular container made of, e.g., copper, in which a working fluid is sealed is also known. In the heat pipe, the working fluid evaporates when the heat pipe is partially heated and the vapor (evaporated working fluid) moves through the pipe to a low temperature section of the heat pipe where the vapor condenses, thus the heat is transported by the working fluid. In particular, a small heat pipe with a diameter of not exceeding 3 mm, which is also referred to as a micro heat pipe, has recently been developed and is in practical use as a cooling device for electronic devices such as laptop computers.
The bore diameter of the heat pipe employed in electronic devices is about as large as 2 mm. Also, it is contemplated that the heat pipe is attached, by fixtures or soldering, to a structure such as a stem, and is used in conjunction with heat dissipation fins or other cooling means. Thus, it is difficult to place the heat pipe in the vicinity of the microscopic light emitting spot of a high-power laser diode and cool the high density heat generated thereby.
Also, the bore diameter of a conventional heat pipe is at least 2 mm and the heat transport capacity of a single heat pipe per unit of cross-sectional area is about 150 to 250 mW/cm2, which is reduced by about ½ to 150 mW/cm2 at the maximum when surrounding structures are included. Also, using fixtures or solder results in an increased contact surface of metals, causing an increased junction thermal resistance. In particular, it is anticipated, in the future, that heat generation density will increase due to the trends toward, for example, higher-power laser diodes, higher density packaging or use of ultraviolet light beams. Therefore, the conventional cooling mechanisms are not appropriate for accommodating these future needs. For example, the electro-optical conversion efficiency of a GaN laser is about 15%, which corresponds to ½ to ⅓ of an infrared laser. Further, the GaN laser has a high light absorption coefficient, limiting the resonator length to 50 to 60% of the infrared laser. In order to obtain a comparable optical power of the conventional 60 W class infrared laser from a GaN laser, a heat dissipation mechanism capable of accommodating heat dissipation density 10 or more times as greater as that of a conventional heat pipe must be realized. Thus, it is necessary to realize a cooling mechanism capable of providing efficient thermal diffusion and a cooling capacity for high density heat at a microscopic spot of the laser diode.